This invention relates to capacitors and in particular to capacitors in which the dielectric is a ceramic material
Ceramic capacitors have been manufactured for several years and have replaced the older types of capacitors, e.g. paper, mica and plastics foil, in numerous applications. Ceramic dielectrics have the advantage of a high dielectric constant and can thus be used to fabricate capacitors of a small physical size.
With the advent of high density printed circuits, thick film circuits and hybrid circuits, there is an increasing requirement for capacitors having higher capacitance/value ratios than are available at present even in the ceramic field. It is well known that the capacitance of a capacitor can be increased in one of two ways. Either the thickness of the dielectric material between the capacitor plates can be reduced or a material of higher intrinsic or effective dielectric constant can be used. In the case of ceramic capacitors, present manufacturing techniques are approaching the limit of "thinness" of the dielectric beyond which the risk of pinholes leading to breakdown between adjacent electrodes becomes unacceptable
Techniques have therefore been investigated for increasing the effective dielectric constant of the ceramic material. In particular it has been found that doping of the ceramic grain boundaries results in a substantial increase in the effective dielectric constant and hence in the capacitance value of the capacitor. This technique has not, however, been employed in the commercial production of ceramic capacitors as it has been found that, during the manufacturing process which involves relatively high temperatures, the dopant materials react with the electrode materials with consequent adverse effect on the electrode characteristics of the capacitor.
Ceramic capacitors are usually made by spreading a thin smooth coating of paste containing a ceramic, a binder and a solvent onto a smooth non-absorbent surface. The solvent is allowed to evaporate leaving a thin coherent sheet of the "green" ceramic material. Electrodes are screen printed onto the sheet material which is then subdivided, stacked into multilayers and fired at a high temperature thereby forming the ceramic. It is only after this firing process that doping of the ceramic grain boundaries can be effected in the conventionally manufactured ceramic capacitors by a process involving a second firing at a somewhat lower temperature. It is during this second firing process, typically at 1000.degree. to 1300.degree. C., that reaction of the dopant materials with the electrodes takes place with consequent degradation of the properties of the finished capacitor.